Process for improving the resistance to corrosion of aluminum base alloys



Dec. 8, 1936. A. BECK 2,063,022

PROCESS FOR IMPROVING THE RESISTANCE TO CORROSION 0F ALUMINIUM BASE ALLOYS Filed Dec. 25, 1933 A420 fleck 9; lnven for By Ahorn s ent-whether deliberately added or notbut alsov Patented Dec. 8, 1936 UNITED STATES PRQCESS FOR IMPROVING THE RESISTANCE T0 CORROSION ALLOYS OF ALUMINIUM BASE Adolf Beck, Bitterfeld, Germany, assignor to I. G. Farbenindustrie Aktiengesellschatt, Franktort-on-the-Main, Germany Application December 23, 1933, Serial No. 703,756

In Germany December 24, 1932 4 Claims.

This invention relates to aluminium base alloys and more particularly to aluminium base alloys containing suflicient magnesium not only to combine with any further alloying constituents presin excess of that further required to form at room temperature, a saturated solid solution with all the basic aluminium. The quantity of magnesium which is thus available for forming a solid solution with the basic aluminium, in a state uncombined with any foreign constituents present, will be hereinafter referred to as available magnesium. Thus, in aluminium base alloys containing other elements such as silicon (usually pressent either as an impurity or as an intentional constituent in most aluminium alloys) or zinc, which combine more readily with magnesium than does aluminium, the alloys to which the present invention relates are only such as contain at least sufficient magnesium not only to enter into combination with such other elements (to form for example MgzSi, etc.) but also to form at room temperature a saturated solid solution with all the aluminium present. This means in effect that the alloys contemplated by the invention must contain more than about 3 percent of magnesium in combination with the alminium base metal, and since other elements, such as silicon, are always present in commercial aluminium in quantities of the order of one tenth of one percent the minimum magnesium content of the alloys contemplated by the invention (based on the foregoing considerations) will in practice be about 4 percent.

In alloys of this kind which have been caused to solidify from the molten or fused state by allowing them to cool in the ordinary manner, the micrographic structure is found to consist substantially of two main constituents or phases,

namely of large a-crystals consisting of an unsaturated solid solution of the compound AlaMgz in aluminium, and of a coarse network of smaller particles of fi-type crystals surrounding the said lit-crystals and probably consisting of a solid solution of minor quantities of aluminium in the com-' pound AlaMgz. However, the texture thus formed is not inphase-equilibrium, and it is therefore possible to establish the actual equilibrium by an annealing process allowing intercrystalline diffusion to take place in a manner known per se and, as applied to alloys of the system Al-Mg, already-described by Hansom and Gaylor in the Journal of the Institute of Metals XXIV (1920) pp. 201-232. In this manner, the a-crystals are enabled to take up further quantities of Al:Mgz--- (Cl. Mil-21.1)

compound from the intercr'ystalline network up to their limit of saturation, and, by applying sufflciently high temperatures-ranging between about 250 and about 450 C. and depending on the magnesium content of the alloy-for a suflicient length of time, the whole texture of the alloys may be reduced to an entirely homogeneous mass of a-crystals which are more or less saturated with respect to magnesium, according to the percentage of the latter element which is present in the alloy.

In the annexed drawing, the phase diagram of the binary system Al-Mg up to 20 percent magnesium as published in Naturwissenschaften 20 (1932) p. 533 is reproduced. It appears from this diagram that homogeneous solid solutions consisting solely of a-crystals. as aforesaid can be obtained up to a content of about 15 percent of available magnesium in the alloys, and that in order to obtain such homogeneous solid solutions from the (heterogeneous) alloys as cast, the application of heating temperatures ranging from between about 250 C. up to about 450 C. and depending on the magnesium content is necessary.

It has also been stated that aluminium containing up to 10 percent of magnesium when having a completely homogeneous texture of this kind, displays an increased resistance to corrosion as compared with the alloys as cast, in which the texture is coarsely heterogeneous (of. U. S. Patent specification No. 1,742,557). By quenching the thus treated alloys or allowing them to cool rapidly to room temperature this homogeneous texture may be retained also under normal temperature conditions, although the a-crystals at that temperature when containing more than about 3 percent of available magnesium are oversaturated with respect to magnesium, since the solubility of the latter element in solid. aluminium strongly decreases with diminishing temperatures.

It has now been ascertained that by subjecting the homogeneous alloys consisting of oversaturated a-CIYStBJS to an annealing process at temperatures below the line dividing the fields representing u-crystals, and (oz-H8) -crystals, respectively, in the annexed diagram and which are thus lower than those at which the alloys were previously homogenized, but which are at the same time sufliciently high to allow of intra-crystalline segregation (i. e. from about 200 C. upwards), the magnesium content of the a-crystals exceeding the limit of saturation. at that temperature is caused to segregate from within the a-crystals in a highly dispersed form; that is to say, every individual u-crystal becomes interspersed with a large number of minute particles of the p-type which is rich in AlaMga-compound. This texture of course is again heterogeneous, but is distinguished from the heterogeneous texture of the alloys as cast in that the ,B-type constituent is no longer coarsely distributed in a network surrounding the a-crystals, but is highly'dispersed in a uniform manner throughout and within the said a-crystals; when viewed with the microscope. a texture of this kind, after the application of suitable etching agents ofi'ers the mottled appearance which is well known as characteristic of a segregate in a highly dispersed form.

As stated above, the view has hitherto been held that to render aluminium base alloys containing between 3 percent and 10 percent magnesium corrosion resistant, it is advisable that their texture be homogeneous. It has now been ascertained as a fact which forms the basis of the present invention that the homogeneous texture by no means displays the maximum resistance of corrosion of the alloys in question, and that the alloys having a heterogeneous texture provided the p-component is present in the highly dispersed form as produced for instance by annealing an oversaturated homogeneous solid solution within the temperature range set forth above- -ofier superior resistance to the action of corroding agents such as sea-water, and undergo,

a considerably lesser reduction of their mechanical strength properties when in prolonged contact with such corroding agents, particularly in the cold-worked alloys. This statement applies not only to the alloys containing between about 3 percent and 10 percent magnesium, but is also true when the magnesium content of the alloy is increased up to about 16 percent, and it extends to the whole range of alloys as defined in the introductory paragraph of this specification.

The term corrosion as employed above does not only apply to an externally and macroscopically manifest corrosion, for example by discoloration of the surface of the metals, but particularly also to intercrystalline corrosion phenomena which in course of time lead to a decrease ln the mechanical strength properties. The intercrystalline corrosion is particularly conspicuous in the alloys after their having been subjected to a plastic deformation in the cold, for instance by a forging or rolling process. Ac-

' cording to present day views such intercrystalline corrosion is in fact considered far more dangerous from a technical point of view because of the lowering of the mechanical strength values of the material produced thereby; and its progress is therefore usually followed by determining the progressive decrease in tensile strength etc. after progressively prolonged contact of the alloys with corroding media.

In accordance with the present invention the material is subjected to a thermal treatment.

' bility, in the solid state, of magnesium in aluminium becomes noticeable with decreasing temperature, the latter point in alloys containing about 3 percent of available magnesium being represented by a temperature of about 250 C. and being shifted to higher temperatures up to about 450 C., as the .magnesium content increases. The thus treated alloys are thereupon, preferably after chilling, subjected to an annealing process at temperatures which are below the lower limit of the temperatures employed in the first heating stage, but are still sumciently high to allow of intracrystalline segregation. This annealing results in a segregation, in highly dispersed form, of particles of thefi-type which are rich in magnesium as compared with the. basic a-crystals and probably consist of the compound AlaMgz.

To promote the efliciency of and/or to accelerate the first stage of the proces it is advisable to subject the alloys, prior to the thermal treatment, to a plastic deformation which to some extent at least already leads to a disintegration and distribution of the coarse-grained lumps of segregated ,B-component present in the alloys as cast.

According to a modification of the present invention the desired effect can be equally attained by substituting, for the second or annealing process an artificially strongly retarded cooling, preferably down to at least 100 C., which immediately follows the first or heating stage. The fact that an artificially retarded cooling process produces the same result as the quenching and reheating of the homogenized alloys is due to the circumstance that in the alloys in question and at the temperatures concerned the speed of diffusion. is exceedingly small, and that, in order 'to produce segregation, it is only necessary to maintain the alloys at the temperatures of segregation for an extended period of time; this can obviously be eilected by artificially retarding the natural cooling process just as well as by quenching the alloy and then reheating it.

The process according to the present invention initsapplication, not limited to the binary alloys of aluminium containing between about 3 and 10 percent magnesium, but also extends to similar alloys containing up to about 16 percent of magnesium and furnishes particularly conspicuous advantages when manganese in quantities ranging between 0.1 and 2 percent is present besides. Silicon, nickel. cobalt, chromium, cadmium and similar elements may also be present in the alloys in small quantities whether tie-- liberately added or not.

The present invention is particularly useful when applied to alloys which are cold worked, either before or after the thermal treatment according to the nvention, since when the said thermal treatment is omitted the texture of these alloys is extremely liable to be aflected by cracking and splitting, and consequent destruction, in contact with corroding media.

Example 1 The effectof this heat treatment upon the mechanical strength values of the alloys after progressive times of contact with sea-water, as compared with the same alloy which had only been homogenized at 450 C. for 24 hours is represented by the following table. The contact with sea-water was efiected in the recognized manner by subjecting the alloys in sheet form to a spraying with sea-water three of four times per day (sea-water spray-test).

Example 2 I The same alloy as cast was subjected to a heating at 450 C. for 24 hours, then quenched, and subsequently rolled at about 350 C. (the final pass involving a reduction by 10 percent being carried out in the cold) into sheets of 1 mm. thickness. The sheets were then annealed at 280 C. for about 2 hours.

The comparative sea-water spray test yielded the following results.

After months Annealed alloy (according to the invention):

Tensile strength kg/sqmm 34 33 37 34 31 Elongation percent 8 9 B 5 4 D Homogeneous alloy.-See Example 1.

I claim:-

1. A process for improving the resistance to corrosion or aluminium base alloys containing available magnesium in percentage quantities of from about 3 to about 16 which comprises sublooting said alloys in the as cast state to a heat treatment at a temperature ranging from about 250 to about 450 C. Ior such time as to impart a homogeneous crystalline structure to said alloys, quenching said alloys and re-heating said alloys to a temperature range the upper limit of which for varying percentages of magnesium in the alloys is represented by the boundary line separating the a-crystalfrom the a-l-e-crystalfield in the annexed phase diagram and the lower limit of which is represented by a temperature in excess of 200 C.

2. The process as defined in claim 1 in which the alloys in the as cast state are first plastically worked to produce a disintegration and distribution of the coarse grained p-crystais.

3. The proces as defined in claim 1, wherein the alloy contains 8.8 per cent of magnesium. wherein the first heat treatment takes place at a temperature of about 450 0., wherein the quenching is efiected in water and wherein the final heat treatment takes place at a temperature of about 320 C.

4. The process as defined in claim 1 wherein the alloy contains 8.8% of magnesium, wherein the first heat treatment takes place at a temperature of about 450 C. and the final heat treatment at a temperature of about 280 C. and wherein a hot rolling at a temperature of about 350 C., the final pass of which-involving a reduction 01 about 10% is carried out 'in the cold, is interpolated between the quenching and final heat treatment steps.

ADOLF BECK. 

